Answer PostgreSQL core database questions by researching the local postgres source tree, PostgreSQL documentation under doc/src/sgml, relevant source code, and DeepWiki repository knowledge for postgres/postgres; use when the user asks any PostgreSQL database behavior, internals, SQL, configuration, administration, performance, storage, planner, executor, replication, or documentation question and wants a sourced Markdown answer saved to the current project's markdown directory. Refuse questions about non-PostgreSQL-core projects, including PostgreSQL ecosystem plugins, extensions maintained outside postgres/postgres, external tools, drivers, cloud services, forks, or third-party products.